Acoustic devices such as headphones can include active noise reduction (ANR) capabilities that block at least portions of ambient noise from reaching the ear of a user. A single feedforward microphone is favored in many acoustic devices because it is low-cost and easy to implement. The performance of these devices can be estimated in terms of a level of coherence between the noise signals at the positions of the microphone outside of the devices and a virtual microphone inside the devices (e.g., a user's ear). The coherence of these devices, however, may be degraded when there are noise signals from multiple noise sources that cannot be captured by a single feedforward microphone.